Arrows use the "damage" field as a multiplier to calculate their actual dealt damage. Tridents also have this field, but it seems to have no effect - they always do 4 hearts of damage.

To reproduce, set up a repeating command block with:

/execute as @e[type=trident] run data merge entity @s {damage:0d} (or any other damage amount)

Now throw tridents at mobs. The trident will still deal 4 hearts.